build(android): split GPU acceleration into optional features

Both `kon-transcription` and `kon-llm` previously hardcoded their native
acceleration features in Cargo.toml — `whisper-rs` with `vulkan`,
`llama-cpp-2` with `openmp` + `vulkan`. That worked everywhere desktop
ships (Linux/macOS/Windows all have Vulkan via MoltenVK on Mac), but it
made an Android build structurally impossible: NDK builds against drivers
that vary wildly across SoCs (Adreno OK, Mali patchy, PowerVR worse), and
some older devices have no Vulkan at all.

Roadmap step 0 from the Android plan: make the GPU acceleration
opt-in so a CPU-only target compiles. Reuses the existing pattern that
README's "future Windows non-AVX2 build" comment hinted at.

- kon-transcription: new `whisper-vulkan` feature gates `whisper-rs/vulkan`
  via the optional-syntax `whisper-rs?/vulkan`. Default features stay as
  `["whisper", "whisper-vulkan"]` so desktop is unchanged.
- kon-llm: new `gpu-vulkan` and `openmp` features each gate the matching
  `llama-cpp-2` feature. Default stays `["gpu-vulkan", "openmp"]`. They are
  independent so an Android Vulkan build can opt into vulkan without
  openmp (NDK OpenMP linking has known cross-version fragility).

CPU-only build invocations:
  cargo build -p kon-transcription --no-default-features --features whisper
  cargo build -p kon-llm --no-default-features

Verified: all 91 tests in the buildable-in-sandbox crates still pass.
The two crates whose Cargo.toml changed (kon-transcription, kon-llm)
can't be compiled in this sandbox (ort-sys CDN + cmake-built llama.cpp);
CI's Linux/macOS/Windows builders will exercise the default-feature path
exactly as before.

https://claude.ai/code/session_0189xUb6ie6t9qHkzatGZ9Rb

crates/transcription/Cargo.toml

@@ -6,13 +6,19 @@ description = "Speech-to-text engine wrappers, model management, and inference c
 build = "build.rs"
 
 [features]
-# Whisper backend (direct whisper-rs, vulkan-accelerated). Default on —
-# gating it exists so a future Windows non-AVX2 build, or a cloud-only
-# ASR configuration, can drop whisper-rs-sys entirely per brief item
-# #13. Disabling this feature also drops the WhisperRsBackend module
-# and the load_whisper entry point.
-default = ["whisper"]
+# Whisper backend (direct whisper-rs). Default on — gating it exists so
+# a future Windows non-AVX2 build, or a cloud-only ASR configuration,
+# can drop whisper-rs-sys entirely per brief item #13. Disabling this
+# feature also drops the WhisperRsBackend module and the load_whisper
+# entry point.
+#
+# `whisper-vulkan` is a separate feature so a non-Vulkan target (Android
+# without GPU drivers, a CPU-only Windows build) can pull in whisper-rs
+# but skip the Vulkan backend. Build CPU-only with:
+#   cargo build -p kon-transcription --no-default-features --features whisper
+default = ["whisper", "whisper-vulkan"]
 whisper = ["dep:whisper-rs", "dep:num_cpus"]
+whisper-vulkan = ["whisper-rs?/vulkan"]
 
 [dependencies]
 kon-core = { path = "../core" }
@@ -30,8 +36,9 @@ futures-util = "0.3"
 # Download integrity verification
 sha2 = "0.10"
 
-# Gated behind the `whisper` feature (see [features] above).
-whisper-rs = { version = "0.16", default-features = false, features = ["vulkan"], optional = true }
+# Gated behind the `whisper` feature (see [features] above). Vulkan is
+# additive via the `whisper-vulkan` feature so non-GPU targets can drop it.
+whisper-rs = { version = "0.16", default-features = false, optional = true }
 
 # Direct whisper-rs backend (WhisperRsBackend): thread pool sizing.
 # Gated alongside whisper-rs since no other code in this crate needs it.